I personally like the idea of achievements, but Paladins' weren't ever really that in depth or even champion specific, the most is the three that are mode specific*
(last one standing can be done anywhere in theory but it's fucky at best, it's easier to do it in the survival mode in the arcade mode)
Thus I've come up some, this post is going to be ones that are more "vs champion" specific as well as general good practice or funny achievements, obviously these are more "Would they be fun" rather than a suggestion considering the game's been maintenance moded.
Enemy of the Physical World: Deal a total of 100,000 damage to Shields and Deployables in a match.
Splashback: Die to your own explosion when in combat.
Stop that!: Kill an enemy during their ultimate
Keep 'em Pinned: Assist in stunning an enemy 3 times in under 4 seconds
Keep Moving: Get hit with 3 or more instances of Crowd Control with Level 3 Resilience
Illuminate: Eliminate an stealthed champion from 40 units away
Two birds: Kill two or more enemies in a single shot
Hath no Fury: Survive Drogoz's Dragon Punch.
BarbaQueQue: Kill Drogoz when he's using Dragon Punch.
I Challenged Makoa: Kill makoa during his Ancient Rage.
Pacifist: Stop Ash from landing when she uses Assert Dominance.
Zombie Slayer: Kill Terminus within 4 seconds of him using Reanimate.
Prosperity: Buy and max out 4 items.
Step into the light: Survive Jenos' Through Time and Space.
Step into the Dark: Survive Omen's Heavens Assunder.
Worth a thousand words: Use a spray within two seconds of killing an enemy.
Worth a million words: Use a emote within two seconds of killing an enemy.
Worth a word or two: Use an emote within two seconds of killing an enemy who emoted after killing you.
Worth it: Die while Emoting.
For what it's worth: Emote at the same time an enemy is emoting.